Create a typedef for the image type to be loaded

This will allow an ImageLoader to be used as an abstract type
Loader<LoadableImage> instead of its concrete class type.
Fix missing file extension

Not quite sure how I managed to commit a file without its .hx extension,
but I did.
Merge pull request #16 from massiveinteractive/feature/loadable-image-typedef

Create a typedef for the image type to be loaded